1. Field of the Invention
The present invention relates to a traveling wave tube having a helix which causes a high frequency signal input from the outside and an electron beam to interact with each other.
2. Description of the Related Art
FIG. 1 is a schematic longitudinal sectional view of the construction of a traveling wave tube disclosed in Japanese Patent Laid-Open No. 2005-339892 (hereinafter referred to as patent document 1).
As shown in FIG. 1, traveling wave tube 1 has electron gun 10 which emits an electron beam, high frequency circuit unit 20 which amplifies a high frequency signal (microwave), collector electrodes 30 used to capture the electron beam, anode electrode 40 used to draw out electrons from electron gun 10 and to guide the electron beam emitted from electron gun 10 to high frequency circuit unit 20.
Electron gun 10 has cathode electrode 11 for emitting electrons, heater 12 for supplying thermal energy for causing emission of thermal electrons from cathode electrode 11 and Wehnelt electrode 13 used to focus electrons emitted from cathode electrode 11
High frequency circuit unit 20 has helix 21 which causes an electron beam emitted from electron gun 10 and a high frequency signal (microwave) to interact with each other, input circuit 22 for supplying helix 21 with a high frequency signal input from the outside and output circuit 23 for outputting the high frequency signal output from helix 21 to the outside. A waveguide or a coaxial line for transmitting the high frequency signal is connected to input circuit 22 and output circuit 23. FIG. 1 shows an example of the construction in which waveguide 50 is connected to input circuit 22 and output circuit 23.
Each of input circuit 22 and output circuit 23 has coaxial inner conductor 24 for input or output of the high frequency signal to or from helix 21. The high frequency signal is input to helix 21 via coaxial inner conductor 24 provided in input circuit 22. The high frequency signal output from helix 21 is radiated from coaxial inner conductor 24 provided in output circuit 23 into waveguide 50 connected to output circuit 23.
Helix 21 is supported and fixed in tubular case 26 by (ordinarily three) supporting columns 25 made of a dielectric or the like. Coaxial inner conductor 24 is fixed at a predetermined position on the sealed-end side of waveguide 50 by a ceramic window provided for vacuum-sealing the interior of case 26.
FIGS. 2A and 2B show a connection between a coaxial inner conductor and a helix disclosed in Japanese Patent Laid-Open No. 5-41175 (hereinafter referred to as patent document 2). FIG. 2A is an enlarged longitudinal sectional view showing an essential high frequency circuit unit, and FIG. 2B is a sectional view of the coaxial inner conductor taken along line A-A′ in FIG. 2A.
In ordinary cases, a high-melting point metal such as molybdenum or tungsten is used for helix 21 and coaxial inner conductor 24. It is difficult to directly weld helix 21 and coaxial inner conductor 24 made of such a metal. Therefore, as shown in FIG. 2A, helix 21 and coaxial inner conductor 24 are connected by using metal tape 27 which is made of nickel, platinum or an alloy containing nickel and platinum, and which is weldable to helix 21 and coaxial inner conductor 24. One end of metal tape 27 is welded to an end portion of helix 21 in advance, and another end of metal tape 27 is laser-welded to an end portion of coaxial inner conductor 24.
Coaxial inner conductor 24 is columnar in ordinary cases. Therefore, patent document 2 proposes a construction in which the end portion of coaxial inner conductor 24 is chamfered into a flat surface, as shown in FIG. 2B for example, and the flat surfaces of metal tape 27 and coaxial inner conductor 24 are brought into contact with each other to increase the area of contact between metal tape 27 and coaxial inner conductor 24.
FIGS. 3A and 3B are diagrams showing welded portions of metal tape 27 and coaxial inner conductor 24 in the related art shown in FIGS. 2A and 2B.
As described above, helix 21 is supported and fixed in case 26 by supporting columns 25. Therefore, an operation to weld metal tape 27 to coaxial inner conductor 24 is performed from the opening side of case 26 (in direction A in FIG. 3A).
More specifically, metal tape 27 formed into an L-shape is joined to an end portion of helix 21 in advance so that the upright surface of metal tape 27 can be seen from the opening side of case 26. Helix 21 to which metal tape 27 has been joined is supported and fixed in case 26 and laser welding for welding an end portion of metal tape 27 to coaxial inner conductor 24 is thereafter performed from the opening side of case 26, as shown in FIG. 3B.
In laser welding for joining metallic members by using laser beam as a heat source, a portion called a nugget is formed as a result of melting and solidification of a metal is formed in a portion irradiated with laser beam and its surrounding portion. A plurality of solid round marks shown at an end of metal tape 27 in FIGS. 3A and 3B represent welded portions (nuggets) of metal tape 27 and coaxial inner conductor 24 thus formed.
In the high frequency circuit unit in the related art, as shown in FIGS. 3A and 3B, the welded portion is limited to the end of metal tape 27 as the boundary between metal tape 27 and coaxial inner conductor 24 which can be visually checked from the opening side of case 26. Therefore, the high frequency circuit unit in the related art has a problem in that the area of contact between metal tape 27 and coaxial inner conductor is reduced.
If the area of contact between the metal tape and the coaxial inner conductor is reduced, the thermal conductivity at the portions of the metal tape and at the coaxial inner conductor that are in contact with each other becomes lower and, correspondingly, the heat dissipation capacity with respect to heat generated in helix 21 becomes lower to allow the temperature of helix 21 to rise. With this change, there is a risk of a deterioration in electrical characteristics of the traveling wave tube and a reduction in stability of the operation of the traveling wave tube. In the case of a high-output traveling wave tube having an increased amount of heat generation in helix 21 in particular, there is a possibility of a breakage in the traveling wave tube, such as melting and cutting of metal tape 27 at the worst.
According to the above-described patent document 2, the end portion of coaxial inner conductor 24 is chamfered into a flat surface and the flat surfaces of metal tape 27 and coaxial inner conductor 24 are brought into contact with each other to increase the area of contact between metal tape 27 and coaxial inner conductor 24. Even in such a construction, the welded portion is limited to the end of metal tape 27 as the boundary between metal tape 27 and coaxial inner conductor 24 which can be visually checked from the opening side of case 26. In such a case, since effective contact between metal tape 27 and coaxial inner conductor 24 is not ensured at any position other than the position of the welded position, there is a possibility of failure to sufficiently increase the contact area even when the construction disclosed in patent document 2 is used.